1. Field of the Invention
This invention relates to fluid filters and, more particularly, to techniques for removing magnetizable material from fluids through electromagnetic techniques and the like.
2. Description of the Prior Art
Iron oxides and other substances that are attracted to magnets can be removed from fluids by means of this particular property. One illustrative method that relies on this particular phenomenon is described in more complete detail in H. G. Heitmann et al. U.S. Pat. No. 3,979,288 titled "Double-Flow Magnetic Filter, Apparatus and Method". In this Heitmann et al. patent a generally cylindrical vessel that contains a charge of magnetizable balls in the central portion of the vessel has a circumscribing electromagnet coil. Fluid that is to be filtered is admitted to the vessel through tubes that discharge into the central plane of the charge of balls by way of an array of open passages.
Upon energizing the electromagnet coil, a magnetic field is produced that magnetizes the balls. Magnetic impurities are attracted to these balls, thereby enabling purified water to flow from the vessel.
There are, however, a number of disadvantages that characterize this technique. Frequently, and particularly in connection with large commercial nuclear power plants, it is desirable to operate the electromagnet filter at one half capacity. This apparatus can not be operated in a one half capacity status, but must be operated at full capacity.
Naturally, devices of this nature also must be flushed from time-to-time. When flushing a filter that has been used to purify water in a nuclear power plant there is a further difficulty if the contaminants that are to be washed from the surfaces of the balls are radioactive. In this instance, for economy and for safety in radioactive waste material disposal, it is clearly desirable that the combined volume of flushing fluid and entrained or dissolved radioactive contaminants should be as small as possible. This need exists because the costs and difficulties associated with radioactive waste disposal are frequently related to the volume that must be processed. The volume of discharged flushing fluid from prior art filters nevertheless is rather large.
There also are a number of further disadvantages that have been observed in connection with devices of the foregoing character. Thus, for example, the pressure loss that is experienced in discharging the contaminated fluid that is to be filtered into the center of the bed of magnetizable balls is quite high. The balls that form the bed frequently block the passages into the filter bed thereby impeding the flow of liquid into the bed and producing a rather poor flow distribution within the filter bed.
This combination of single filter bed and circumscribing electromagnetic coil also imposes rather inflexible design limitations. Typically, the depth of the filter bed is not chosen to match the optimum flow length for the filtrate through the bed, but is determined in response to other, less desirable criteria. Typical among these undesirably dominant criteria is the maximum acceptable pressure loss in the filter bed and the practical considerations that essentially fix the electromagnetic coil length.
Cleansing the balls in the filter bed during flushing also is less than satisfactory because the flow distributor blocks a portion of the bed. Hence, the bed does not experience a beneficial longitudinal fluidization or "rising" movement. If the filter bed could be fluidized, however, it should provide a number of advantages during the flushing and cleansing operations. Fluidization, for instance, would cause the filter bed to expand and loosen debris that otherwise would be trapped in the bed. The balls also would become more fully exposed for improved washing in these circumstances, and also beat against one another to further shed contaminating matter.
The array of relatively small diameter tubes that establish flow into and out of the vessel also becomes subject to a number of other possible difficulties. Vibration and the fatigue and ultimate metal failure that often accompany fibration forces are illustrative in this respect of these difficulties that are likely to be aggravated in these prior art filters.
Accordingly, there is a genuine need for an electromagnetic filter that overcomes these serious industrial problems.